Radio-frequency oscillators are intended to operate in high frequency ranges, typically between a few tens of megahertz and a few tens of gigahertz.
To meet the demands arising from the development of nomad applications, and particularly mobile telephony (cell phones, mobiles), because of the saturation of the frequency bands assigned to telecommunications, a proposal has been made to replace the static allocation of said frequency bands with a dynamic allocation. This principle rests on the capacity to analyze the frequency spectrum, and identify free frequency bands, in order to be able to use them. This is then known as “opportunistic radio”.
However, in order to apply this dynamic frequency allocation principle, the devices that use it must be provided with very wide band oscillators, and furthermore be highly effective in terms of phase noise, and therefore have a high quality coefficient Q=f/Δf.
One technical solution suitable for meeting these demands lies in using radio-frequency oscillators based on spintronics using the spin transfer phenomenon. With oscillators of this kind wide frequency accordability is provided in a frequency range running from a few hundred MHz to a few tens of GHz offering a high quality factor Q, and employing a relatively straightforward architecture.
Spintronics uses electron spin as an additional degree of freedom compared with conventional electronics which uses only the electron charge, in order to generate new effects.
It has thus been shown that a spin polarized current passed through a thin magnetic layer can induce a reversal of its magnetization in the absence of any external magnetic field. This phenomenon is known as spin transfer. In some geometries, the spin polarized current may also generate sustained magnetic excitations, also referred to as oscillations. The use of the effect of generating sustained magnetic excitations in a magnetoresistive device allows this effect to be converted into an electric resistance modulation that can be used directly in electronic circuits, and is therefore as a result able to intervene directly at frequency level.
Document U.S. Pat. No. 5,695,864 describes various developments that employ the aforementioned principle, and in particular describes the precession of the magnetization of a magnetic layer passed through by a spin polarized electric current. A stack of magnetic layers able to constitute such a radio-frequency oscillator has been shown in FIG. 1. This stack is inserted between two electrical contact zones 5, 5′, made for example out of copper, aluminium or gold.
The layer 2 of this stack, known as the “trapped layer”, is magnetized in a fixed direction. It may consist of a single layer, with a typical thickness of between 5 and 100 nanometers, made of cobalt for example or more generally of a cobalt- and/or iron- and/or nickel-based alloy (for example CoFe or NiFe). This trapped layer 2 may be single or synthetic anti-ferromagnetic, in other words consist of two ferromagnetic layers coupled anti-ferromagnetically through a spacer of appropriate composition and thickness, made for example of ruthenium with a thickness of between 0.6 nm and 0.9 nm. This trapped layer fulfils the function of polarizer. As such, the electric current electrons passing through the layers constituting the magnetoresistive device perpendicular to their plane, reflected or transmitted by the polarizer, are polarized with a direction of spin parallel to the magnetization that the layer 2 has at the interface opposite the one in contact with an anti-ferromagnetic layer 6, with which it is associated, and intended to fix the direction of its magnetization. This anti-ferromagnetic layer 6 may for example be made of IrMn or FeMn or PtMn or NiMn.
The trapped layer 2 receives on its face opposite the face receiving the anti-ferromagnetic layer 6 another non-magnetic layer 3 functioning as spacer. This layer 3 is metallic in nature, typically a layer of copper between 3 and 10 nanometers thick, or is constituted by a fine insulating layer of the aluminium oxide type, with a typical thickness of between 0.5 and 1.5 nanometers, or of magnesium oxide, with a typical thickness of between 0.5 and 3 nanometers. On the other side of the spacer 3 is put in place a layer 1 known as a “free layer”, generally narrower than the layer 2. This layer may be single of composite, in other words formed by the association of a plurality of magnetic layers. It may also be coupled with an anti-ferromagnetic layer 4 added to it on its face opposite the interface of the layer 1 with the spacer 3. It must simply remain freer than the trapped layer. This layer 4 is constituted for example of an alloy such as Ir80Mn20, FeMn or PtMn. The material of the layer 1 is generally constituted of a cobalt- and/or iron- and/or nickel-based alloy.
A proposal has been made to produce the magnetoresistive stack of the system so described in the form of a cylinder, thus constituting a nanopillar, with the critical diameter thereof being between 50 and 200 nanometers. As such, with said configuration and under the sole action of a spin-polarized direct current passing through, in other words perpendicular to the stack of its magnetic layers, the magnetization of the free layer is made subject to sustained oscillations under certain conditions. The sustained magnetization oscillations appear more often than not when the effects of the effective magnetic field acting on the magnetization of the free layer and the effect of the spin polarized current passing through this layer are antagonistic. This is the case for example if a field is applied parallel to the magnetization of the trapped layer 2, promoting the parallel alignment of the magnetizations of the layers 1 and 2 and if the current flows from the trapped layer towards the free layer (the electrons going from the free layer towards the trapped layer) which promotes the anti-parallel alignment of the magnetizations.
This oscillation generates a very-high frequency alternating voltage via the magnetoresistive properties of these systems. Indeed, the resistance of an anti-parallel alignment is greater than for a parallel alignment of the magnetizations of the polarizing and free layers respectively.
The frequency of this voltage is accordable via the direct current density applied through the stack in a range of frequencies running from a gigahertz to a few tens of gigahertz with a quality factor that can reach very high values above 10.000.
If, on paper, such an architecture is expected to give good results, experience shows that when these oscillators are used in frequency syntheses a number of technological obstacles are encountered, the two main ones being the output power, in other words the amplitude of the wanted signal being too weak, and spectral line widths that are too large, hence a low quality factor f/Δf.
In relation to this latter point, it has been possible to show that significant line widths are generally inherent in a lack of coherence in the precession movement of the magnetization. This lack of coherence seems to have two distinct origins:                the low volume of the magnetoresistive stack which renders it more sensitive to the effects of thermal fluctuations in the magnetization as well as to the nanofabrication process (defects at the edge of the stack causing local trapping sites of the magnetization leading to instabilities in the precession movement of the magnetization);        the Oersted field effect or Amperian field (field created by the current flowing in the pillar and governed by the Biot-Savard Law) induced by the applied current which, in a full cylindrical structure, tends to induce a magnetization shear with a singularity at the centre.        
The intended aim of this invention is to propose a radio-frequency oscillator that can overcome the problem related to the lack of coherence of the precession movement of the magnetization.